This invention relates to cranes, and more particularly, to a marine crane incorporating a high speed winch having a hydraulic heave compensating system together with an automatic lift control system, all to minimize dynamic shock loads imposed on the crane during offloading operations.
During the offloading of cargo from a supply ship, marine cranes can become subjected to unusually large, dynamic shock loads as the ship rises and falls in response to crests and troughs of waves. For example, if a lift occurs when the ship and cargo are moving downwardly into the trough of a wave, the dynamic shock load experienced by the crane can be five times or more greater than the normal static load imposed on the crane. Dynamic shock loads may also be imposed on the crane prior to a lift if its hoist rope is caused alternately to slacken and tighten in response to ship movement. An overload creating severe stresses can also develop if the cargo catches on ship rails or other protrusions of the ship superstructure during a lift.
The occurrence of dynamic shock loads during offloading is accentuated by the difficult operating conditions encountered by the crane operator. The operator is generally located in a cab on a pedestal supported high above the ship and must look nearly vertically downwards to see the deck of the ship. Nevertheless, the operator must maintain the crane hook close to the heaving deck of the ship while slings are attached to the load, then take up the slack in the slings as the deck rises, and finally hoist the load at the proper time, preferably close to a crest. Simultaneously, the operator must maintain luff and slew control to keep the hoist rope vertically positioned above the load so that a dangerous pendulum motion does not develop upon hoisting. Under these circumstances it is extremely difficult for the crane operator to judge the rise and fall of the ship and decide the correct moment to lift a load from the heaving deck.
A crane lifting a load on land experiences shock at the moment of lift, and such cranes may be properly designed to cope with these impacts. In contrast, however, the shock loading imposed upon marine cranes is unpredictable and dynamic and may lead to overload situations and eventually to stress failures. It is thus desirable to have an arrangement that reduces the dynamic shock loading imposed upon marine cranes. This would minimize the possibility of the crane toppling from its mountings, damage to the ship, crane, or its load, and injury to personnel.
The prior art has disclosed various arrangements for reducing the dynamic shock loads imposed upon marine cranes. In some of these arrangements, a device such as a shock absorber, pulley nest, or auxiliary winch is suspended from the crane hook to compensate for the heaving deck. See for example, U.K. Patent Application No. 2,006,151A published on May 2, 1979, and an article entitled "Motion Compensator Handles Cargo," published in Ocean Industry, January, 1979, at page 78. These types of devices, however, are fairly large, heavy and cumbersome structures and as a result reduce the lifting capacity and maneuverability of the crane.
Another type of arrangement for reducing dynamic shock impacts on marine cranes involves the use of a dual system of ropes. See for example, U.S. Pat. Nos. 4,180,171, 4,132,387 and 3,753,552. In these arrangements, one rope is used for hoisting and a second rope is attached to the ship or load. The second rope senses the motion of the ship and through a control mechanism compensates for the heave of the ship by keeping the hoisting rope in constant tension. These systems, however, generally use electronic controls, and if there is a general power outage on the platform the electronic controls may become inoperative. This problem may also occur with control systems that use microprocessors to determine the optimum time for lifting the load. Also, with a dual rope arrangement the ropes may easily become tangled as the ship rolls and pitches.
In still another type of arrangement, see for example, U.S. Pat. No. 3,799,505, the hoist rope extends from its hoist winch over the boom point and down around a sheave attached to the hook, and then back up around the boom point to a compensator winch which is separate from the hoist winch. The compensator winch operates to provide constant tension on the hoist rope. However, in these types of systems, the maximum hoist line speed generally cannot keep up with the velocities of heave on waves having large amplitudes. As a result, slack rope may develop during an upward heave. If this condition persists at the crest of a wave, the compensator winch will still be taking in rope when the load falls away. The result is a shock impact which may be quite severe.
Various other arrangements have also been utilized such as hydraulic rams, as described in U.K. Patent Application No. 2,023,530A published on Jan. 3, 1980, and separate winch arrangements on the crane and supply ship, as described in U.S. Pat. No. 4,180,362. However, none of these arrangements are entirely satisfactory, and the present invention has been developed to provide a high speed winch having a hydraulic wave motion compensating system together with an automatic lift control system.